Effect of Fe Oxidation on Fe-Zn Coating Properties of Si-added Galvannealed Steel Sheets

摘要

The oxidation behavior of Si-added steels was investigated under various oxidizing conditions. Fe oxidation was suppressed when the Si oxide formed at the steel/scale interface was SiO_2, but was accelerated when the formed oxide was Fe_2SiO_4. As a result, reduced Fe covered the steel surface and selective oxidation of Si during recrystallization annealing was suppressed. There is no molten Zn wetting problem in the hot-dip process, even when the steel has a high content of Si. Therefore, in an oxidation-reduction process, it is important to control the species of Si oxide formed at the steel/scale interface in the oxidation step and to secure a sufficient amount of Fe oxide by optimizing the temperature and/or atmosphere in the oxidizing conditions. This transformation behavior is convincingly explained by thermodynamic calculation. As selective surface oxidation of Si is suppressed and a Si-depleted zone is formed at the steel subsurface region, the galvannealing rate was accelerated and the Fe-Zn IMC (Inter Metallic Compound) became fine when a sufficient amount of Fe oxide was secured in the oxidation step. As a result, the anti-powdering property of the Si-added hot-dip Zn galvannealed steel was improved, presumably due to the lower galvannealing temperature.